1. Field of the Invention
The present invention is directed to a radio-frequency antenna for a magnetic resonance system, of the type having two end rings and a number of antenna rods, wherein the end rings are arranged concentrically around an antenna axis and are axially offset relative to one another and the antenna rods connect the end rings to one another, and wherein a coupling element can couple radio-frequency electromagnetic energy into the end rings and into the antenna rods in the transmission mode and can couple radio-frequency electromagnetic energy out therefrom in the reception mode.
2. Description of the Prior Art
Radio-frequency antennas of this type are known as birdcage resonators. As a rule, the antenna rods therein are arranged parallel to the antenna axis.
Birdcage resonators have a number of oscillatory modes, each of which can have its own resonant frequency. It is generally advantageous in magnetic resonance applications to tune the individual oscillatory modes to the same resonant frequency. This can be achieved by means of suitable tuning measures that are described, for example, by Christoph Leussler, Jaro Stimma and Peter Röschmann in their conference paper, “The Bandpass Birdcage Resonator Modified as a Coil Array for Simultaneous MR Acquisition”, ISMRM 1997, page 176.
Due to this tuning to the same resonant frequency (namely, the Larmor frequency), mode separation is no longer possible by means of frequency-dependent impedances, neither in the transmission mode nor in the reception mode.